The present invention relates to a thermal transfer sheet, and more particularly to a thermal transfer sheet which is excellent particularly in lubricity and heat resistance and can contribute to an improvement in the durability of a thermal head.
When an output of computers or word processors is printed by a thermal transfer system, use has hitherto been made of a thermal transfer sheet comprising a substrate film and, provided on one surface thereof, a hot-melt ink layer. Further, there is a sublimation thermal transfer system which is excellent in continuous tone and can provide a full color image comparable to a color photograph.
The thermal transfer sheet used in the hot-melt transfer system or sublimation thermal transfer system comprises a substrate film and, provided thereon, a recording agent layer comprising a hot-melt ink or a recording agent layer containing a sublimation dye.
In the above-described thermal transfer sheet, printing and formation of an image are effected by putting a recording agent and a material, on which an image is to be transferred, on top of each other and heating the assembly from the back surface of the substrate film by means of a thermal head.
However, when materials having a poor heat resistance, such as plastic films, are used as the substrate film, the substrate film sticks on the thermal head during printing, which is detrimental to peelability of the thermal transfer sheet from the thermal head and a slip property or gives rise to breaking of the substrate film, etc.
For this reason, a back surface layer having a heat resistance and a slip property is formed on the back surface of the substrate film. In order to improve the heat resistance of the back surface layer, for example, it is possible to form the back surface layer by using a thermosetting resin, a thermoplastic resin having a high-softening point, etc., or to incorporate inorganic particles or crosslinked resin particles having high heat resistance and lubricity into the back surface layer. In order to improve the slip property of the back surface layer, a proposal has been made on the addition of lubricants, such as silicone oil, low-melting wax and surfactants.
Although surfactants, such as phosphoric ester surfactants, have been used as a lubricant having a high lubricity, they had a problem that, when use is made of surfactants having a low heat decomposition temperature, they are decomposed by heat applied by the thermal head to produce a decomposition product which unfavorably deteriorates or corrodes the surface protective layer of the thermal head or deposits as refuse on the thermal head.
It is also known that a slip layer is formed by using a silicone oil or a silicone wax. Since, however, the silicone oil or wax has a low film strength, it is unfavorably scraped by the thermal head being travelled and gathers as refuse on the thermal head, so that good print cannot be provided.
Under the above-described circumstances, the present invention has been made, and an object of the present invention is to provide a thermal transfer sheet provided with a back surface layer capable of reducing the abrasion of the thermal head.
It is known that a back surface layer containing a phosphoric ester surfactant having an excellent lubricity is provided on the contact surface of the thermal head for the purpose of preventing the inhibition of travel of the thermal head and occurrence of breaking or wrinkle of the thermal transfer sheet attributable to sticking of the thermal transfer sheet to the thermal head. Since, however, the phosphoric ester surfactant has 1 to 2 equivalents of an acid radical, it unfavorably corrodes the thermal head. Further, when the quantity of heat from the thermal head becomes large, the phosphoric ester is decomposed, which further lowers the pH value, so that the corrosion abrasion of the thermal head becomes severe. It is a matter of course that the use of a phosphoric ester surfactant converted to a salt type though neutralization is known in the art. Since, however, such a surfactant has a poor lubricity, the stickiness of the thermal transfer sheet to the thermal head cannot be prevented. Accordingly, the second object of the present invention is to provide a thermal transfer sheet provided with a good back surface layer which is excellent in heat resistance, film property, slip property, etc., can provide good travelling of the thermal head and gives rise to neither gathering of refuse on the thermal head nor abrasion of the thermal head.
In the above-described thermal transfer sheet, since a thin polyester film has hitherto been used as the substrate film, the thermal transfer sheet had a problem that, during printing with a thermal head, the thermal head is fused to the back surface of the thermal transfer sheet to give rise to wrinkle on the thermal transfer sheet or breaking of the thermal transfer sheet. In order to solve these problems, a layer having a heat resistance and a lubricity has been formed on the back surface of the substrate sheet. For example, a proposal has been made on the incorporation of lubricants having an excellent lubricity, such as silicone lubricants, surfactant lubricants, various types of wax and fatty acids, into a heat-resistant layer (see, for example, Japanese Patent Application No. 137977/1990). This method, however, has a problem of contamination of the thermal head with the lubricant bled out on the back surface layer and a problem of poor workability.
Examples of other methods known in the art include a method wherein particles or the like are added to the back surface layer to render the surface of the back surface layer uneven, thereby reducing the area of contact of the back surface layer with the thermal head to impart the lubricity to the back surface layer (see, for example, Japanese Patent Application No. 145390/1990) and a method wherein use is made of a combination of this method with the above-described methods (see, for example, Japanese Patent Application No. 259889/1977). In these methods, however, the travelling of the energized head during energization heating is poor, which often has an adverse effect on the printed image depending upon the configuration of the uneven portions or the properties of the particles used or gives rise to abrasion of the thermal head.
Accordingly, the third object of the present invention is to provide a thermal transfer sheet which is excellent in both the heat resistance and lubricity, can provide good travelling of the thermal head and form a good image and gives rise to no abrasion of the thermal head.